Lens Types

WIDE ANGLE
24mm focal length or lower, fits more into the frame of the image (hence WIDE angle).

ZOOM
A lens that has the ability to change it's focal length (zoom in and out).

TELEPHOTO
Generally 70-300mm zoom, the telephoto lens magnifies the subject at the expense of the frame of vision and the depth of field.

RETRO FOCUS
Difficult to figure out... will revisit .

SUPPLEMENTARY LENS
A simple converging lens that screws on to the actual lens to allow closer focusing.

QUASI FISH-EYE
Extreme wide angle, captures image with all of image frame.

FISH-EYE
Extreme wide angle, captures image circularly within image frame

CATADIOPTRIC
Reflects light back and forth within lens to capture image.

MACRO
Allows very close range focal lengths.

TELECONVERTER
Secondary lens used to increase focal distance of primary lens.

ASPHERICAL LENS

PC LENS / TILT SHIFT LENS
Changes the plane of focus and the perspective.

LENSES CONTINUED (Focal length and Perspective)

As  a rule, lenses do not alter perspective.  In general, any distortion seen in an image is due to the image being viewed from the wrong distance;  view it too close and the perspective appears flat, view it from too far away and the perspective is exaggerated.

This can be proven optically by enlarging the center area of an image taken with a wide angle lens that corresponds with the same area photographed with a longer lens.

VIEWING DISTANCE

A comfortable viewing distance would be equal to a normal reading distance and can usually be calculated by measuring the diagonal of the print.  The correct viewing distance however could be calculated by the focal length of the lens used multiplied by the magnification of the image.

LENSES (refraction of light)

A lens can be thought of a series of prisms which combine to bring the light to a single point or "to focus".  Thus, lenses can be classified as one of two types: Convergent and Divergent:

A Converging Lens bends Incident Rays from infinity so that they intersect at a common point.
A Divergent Lens bends Incident Rays from infinity backwards so that they intersect at a common point. (in this lens, the glass is concave instead of convex)

We call the point at which the rays intersect the "Point of Focus"

REFRACTION (aka. the Pink Floyd Theorem.... or Snell's Law)

The Speed of Light = 299 792 458 m / s

However, through denser mediums such as glass or a beer, the speed of light is slowed down.  If it enters the medium at an angle other than 90 degrees, then the light is bent.  this is called REFRACTION

Refraction of light: 

Blue light is refracted more than Green or Red because these colours are all on different wavelengths.

BLUE >refraction GREEN >refrac RED

as demonstrated by Pink Floyd... 

As you can see red has the least amount of refraction, while green and blue are refracted more. 

REFRACTION INDEX (n) OF MATERIAL:

A figure that represents a material's ability to bend light by slowing it's speed .
Air = 1.00
Glass = 1.5 - 1.8
Beer = 1.34